1. Field of the Invention
The present invention relates to novel mixed rare earth phosphate crystallites well suited for the manufacture of phosphor, and, more especially, to the production of crystallites of certain cerium lanthanum terbium phosphates from water-insoluble rare earth salts, such phosphates being attractive green phosphor precursors.
2. Description of the Prior Art
As early as 1970 it was discovered that the mixed rare earth phosphates, and especially those of lanthanum, cerium and terbium, exhibited an advantageous luminescence property. Thus, numerous cerium lanthanum terbium phosphates, also designated the "LaCeTb phosphates," were developed, containing various concentrations of lanthanum, cerium and terbium.
The processes for their production were classified into two basic categories, namely, "dry route" processes and "wet route" processes.
The dry-route processes, described in particular in JP-62/007,785, WO-82/04,438, JP-62/089,790, JP-59/179,578 and JP-62/000,579, entail phosphating a mixture of rare earth oxides or phosphating a mixed rare earth oxide or a mixed rare earth oxalate via calcination in the presence of diammonium phosphate. The oxides can themselves be prepared by calcining a mixed rare earth oxalate (JP-62/218,477).
The "wet route" processes, described in particular in JP-57/023,674, JP-60/090,287 and JP-62/218,477, entail a direct synthesis of a mixed rare earth phosphate or of a mixture of rare earth phosphates by digestion of a solid compound (carbonate, oxide) with H.sub.3 PO.sub.4 to precipitate the phosphates.
Thus, JP-57/023,674 describes the precipitation of rare earth phosphates by the action of phosphoric acid on rare earth carbonates in dilute medium. The product obtained is dried and then calcined at a temperature of 1,150.degree. C. to provide a product luminescing in the green region of the spectrum. This process requires the use of a dilute reaction medium. As a result of this, it is not applicable on an industrial scale because large volumes of solution would have to be used to produce a small quantity of rare earth phosphate.
Furthermore, these processes entailing the direct precipitation of mixed rare earth phosphates result in the formation of gels or precipitates which filter very poorly.